An attenuated total reflection (ATR)-spectrometer includes a rectangular ATR-crystal that is in contact with a sample to measure a spectrum of the sample. Light is coupled into an end of the ATR-crystal to measure the spectrum. The light propagates by multiple total internal reflections from one end to another end of the ATR-crystal and exits the ATR-crystal there. Due to the total internal reflection, evanescent waves are formed in the sample, wherein an interaction of the evanescent waves with the sample takes place. This interaction leads to a spectrum of the exiting light, wherein the spectrum is characteristic for the sample.
It is disadvantageous that the ATR-crystal is cost-intensive in its production. In addition, the production for a holder for mounting the ATR-crystal in the ATR-spectrometer is cost-intensive. Furthermore, the ATR-crystal has only a small in-coupling surface for coupling the light in the ATR-crystal and a small outcoupling surface for coupling the light out of the ATR-crystal, so that only a small number of light sources and detectors can be provided.
Conventionally, the dimensions of the detectors are in the magnitude of the width of the intensity distributions of the light exiting the ATR-crystal. The intensity distributions conventionally have a Gauss shape with steep gradients. However, this is disadvantageous, since an amount of light that can be measured by the detectors and therefore also the signal-to-noise ratio of the measured spectra strongly depend on the positioning of the detectors. In the case that a calibration of the ATR-spectrometer is carried out, the calibration depends on the amount of light impinging on the detectors. Therefore, the accuracy of the calibration also strongly depends on the positioning of the detectors. A different thermal expansion of the ATR-crystal and of a circuit board, on which the light sources and the detectors are arranged, can possibly lead to a negative impact on the calibration.